Memory in logic cell

ABSTRACT

Methods, devices, and systems for a memory in logic cell are provided. One or more embodiments include using a cell structure having a first gate, a second gate, and a third gate, e.g., a control gate, a back gate, and a floating gate, as a memory in logic cell. The method includes programming the floating gate to a first state to cause the memory in logic cell to operate as a first logic gate type. The method further includes programming the floating gate to a second state to cause the memory in logic cell to operate as a second logic gate type.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.11/821,462 filed Jun. 21, 2007, the specification of which isincorporated by reference herein.

TECHNICAL FIELD

The present disclosure relates generally to memory in logic cells and,more particularly, to cell structure having a first gate, a second gate,and a third gate as a memory in logic cell.

BACKGROUND

Communication bottleneck between memory and logic modules is one of themost serious problems in recent deep submicron very-large-scaleintegration (VLSI) systems. The programmable computing array is morecomplex to build and has lower storage density than a normal memoryarray because of the overhead involved in the storage and logic.

A floating gate MOS transistor is generally used as a memory cell deviceof flash EEPROMs, which can be back-gated. One example of a back-gatedMOSFET transistor is provided in U.S. Pat. No. 7,089,515 entitled,“Threshold Voltage Roll-Off Compensation using Back-Gated MOSFET Devicesfor System High-Performance and Low Standby Power”, issued Aug. 8, 2006.However, the back-gated transistors in the above referenced patent aredirected toward compensating the threshold voltage roll-off.

An example of programmable logic arrays using floating gates is providedin commonly assigned U.S. Pat. No. 6,124,729 entitled, “FieldProgrammable Logic Arrays with Vertical Transistors”, issued Sep. 26,2000. The cells therein have a semiconductor pillar providing a sharedsource and drain region for two separate transistors each havingindividual floating gates and control lines. Whole arrays of suchstructures are field programmed together, versus programming on anindividual cell basis, in order to function as a particular type oflogic plane. In this previous approach, however, the number of connectsbetween planes due to the absence of programmability of each cellindependently and need to connect various entire planes in a certain wayto achieve a desired logic state may add to the complexity and areaconsumed by such a layout.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic illustration of an embodiment of a memory inlogic cell that can be programmed and operated as part of a programmablecomputing array.

FIG. 1B illustrates an embodiment of a memory in logic cell that can beprogrammed and operated as part of a programmable computing array.

FIG. 1C illustrates an embodiment of the cell structure comprising atransistor with a first gate, a second gate, and a third gateconfigurable as a memory in logic cell.

FIG. 2 illustrates a truth table for various first, second, and thirdlogical inputs to the memory in logic cell shown in FIG. 3A.

FIGS. 3A and 3B illustrate a programmable computing array implementingembodiments of the memory in logic cells described in connection withFIGS. 1A-2.

FIG. 4 illustrates an embodiment of a memory in logic computing systemwhere multiple programmable computing arrays are coupled together via aprogrammable routing structure.

FIGS. 5A-5H illustrate an embodiment of the process steps in forming atransistor with a first gate, a second gate, and a third gate to beconfigured as a memory in logic cell in a programmable computing array.

DETAILED DESCRIPTION

Methods, devices, and systems for a memory in logic cell are provided.One or more embodiments include using a cell structure having a firstgate, a second gate, and a third gate, e.g., a control gate, a backgate, and a floating gate, as a memory in logic cell. The methodincludes programming the floating gate to a first state to cause thememory in logic cell to operate as a first logic gate type. The methodfurther includes programming the floating gate to a second state tocause the memory in logic cell to operate as a second logic gate type.

In one or more embodiments, the control gate associated with thefloating gate is used as a first input to the memory in logic cell. Theback gate is used as a second input to the memory in logic cell and thefloating gate is either charged or discharged and used as a third inputto the memory in logic cell. When the floating gate (third input) to thememory in logic cell is charged, the memory in logic cell performs afirst logical operation based on the first and second logical inputsprovided to the control gate and the back gate. When the floating gate(third input) to the memory in logic cell is uncharged, the memory inlogic cell performs a second logical operation based on the first andsecond inputs provided to the control gate and the back gate.

In one or more embodiments, the memory in logic cell is configured as athin semiconductor channel region having a floating gate and anassociated control gate on one side of the channel region along with aback gate on the opposite side of the channel region. The channel regionis configured to a thickness and doping level such that a voltagethreshold (V_(t)) of the channel region depends on the charge on thefloating gate and a potential applied to the back gate. In this manner,the voltage threshold of the channel region is dependent on the floatinggate and is modulated by the back gate.

A memory-in-logic structure in one or more embodiments, such as thatdescribed herein in accordance with one or more embodiments, in whichstorage functions are distributed over a logic-circuit plane to achieveprogrammable computing, can reduce the bottleneck between memory andlogic modules. The memory-in-logic structure allows each individual cellin an array to be programmed to a desired mode, thus allowing forgreater flexibility in configuring an array of cells to a particulardesired state. A memory-in-logic structure provides for a simplifiedcircuit design due to the flexibility associated with each cell beingindividually programmable. Also, there should be an increase inreliability associated with these structures because of the decrease inthe number of connects between cells due to the programmability of eachcell and the lack of the need to connect certain cells in a certain wayto achieve a desired logical state. When a storage function is includedin each cell of a programmable computing array, the array may beregarded as a logic array whose gates and connections can be programmedto realize a desired logical behavior.

FIG. 1A is a schematic illustration of an embodiment of a memory inlogic cell that can be programmed and operated as part of a programmablecomputing array. In the schematic of FIG. 1A, the cell 101 includes astructure having a first gate, a second gate, and a third gate, e.g., acontrol gate 118, a back gate 104, and a floating gate 114. Also, thecell 101 includes a channel region with a source region 119, a bodyregion 110, and a drain region 120.

As shown schematically in FIG. 1A, the cell 101 includes a control gate118 and a back gate 104 formed on opposing sides of the body region 110.A floating gate 114 is shown associated with the control gate 118 and onthe opposite side of the body region 110 than the back gate 104. Oneexample of a method for forming a transistor structure of cell 101 isprovided in U.S. Pat. No. 6,635,923, entitled, “Damascene Double-gateMOSFET with Vertical Channel Regions”, issued Oct. 21, 2003 and havingat least one common inventor. Also, using a back gate to compensate forthreshold voltage is provided in U.S. Pat. No. 7,089,515 entitled,“Threshold Voltage Roll-Off Compensation using Back-Gated MOSFET Devicesfor System High-Performance and Low Standby Power”, issued Aug. 8, 2006and having at least one common inventor.

In contrast to this earlier work, however, as shown in an exampleembodiment in FIG. 1B, the use of a nonvolatile memory cell in aprogrammable computing array with a body region 110 that has a widthwhich is thin, e.g., less than 100 Å, and a doping concentration of lessthan 1×10¹⁷/cm³. For example, one embodiment can have a dopingconcentration of 5×10¹⁵/cm³, which is sufficiently low so that a voltagethreshold (V_(t)) depends on a charge on the floating gate 114 and ismodulated by a potential applied to the back gate 104. For example, whenthe body region 110 has a thickness (“tbod”) of 12 nm and tunnelingoxide thickness (“tox”) of 4 nm, the back gate 104 oxide thickness(“tbgox”) would be 4 nm and the control gate 118 oxide thickness(“tcgox”) would be 12 nm. That is, the voltage threshold (V_(t))necessary to create a channel between the source region 119 and thedrain region 120 on the body region 110 is dependent on the charge onthe floating gate 114 and modulated by the potential applied to the backgate 104.

Hence, in contrast to the above earlier work, one or more embodimentsare such that the respective voltage threshold, e.g., V_(t), and thefloating gate, 114 and back gate 104, of the cell 101 are dependent onthe charge and potential applied to each respectively.

In one or more embodiments, as described in more detail below, thecontrol gate, e.g., 118 serves as a first logical input to the cell 101.The back gate, e.g., 104, serves as a second logical input to the cell101 and the floating gate 114 serves as a third logical input to thecell 101. The cell 101 can be programmed such that the cell 101 performsa first logical operation when the floating gate 114 is in a firststate, e.g., charged, and performs a second logical operation when thefloating gate 114 is in a second state, e.g., uncharged.

FIG. 1B illustrates an embodiment of a memory in logic cell that can beprogrammed and operated as part of a programmable computing array. Inthe example embodiment described in FIG. 1B, the cell structure 101 hasa back gate 104. In this embodiment, the cell structure 101 includes afloating gate, 114, and also an associated control gate, 118. Asmentioned above, the charge on the floating gate, 114, determines thevoltage threshold to turn the transistor on and allow electron flow fromthe source 119 to the drain 120 through the body region 110. The voltagethreshold for the cell structure depends on the charge on the floatinggate 114 is modulated by the back gate 104 input signal, e.g., potentialapplied to the back gate 104, as a second logical input to the cell 101.

In one or more embodiments, which will be described in more detail belowin connection with the truth table shown in FIG. 2, if floating gate,114, is charged negative the cell structure 101, e.g., memory in logicelement, can only be turned on if both the control gate, 118, and theback gate, 104, e.g., first and second logical inputs, have a positivepotential applied thereto, e.g., a positive, modulating second inputsignal to lower the voltage threshold and a positive first input signalsabove the voltage threshold. Alternatively, if the floating gate, 114,is uncharged, either control gate, 118, or the back gate, 104, goingpositive, e.g., either a positive first input on the control gate 118and/or a positive, modulating third input on the back gate 104, can turnthe transistor on. One of ordinary skill in the art will appreciate themanner in which the floating gate, 114, can be charged and chargeremoved therefrom, e.g., either through hot electron injection, FowlerNordheim tunneling, etc. Likewise, one of ordinary skill in the art willappreciate the manner in which exceeding a voltage threshold, e.g.,V_(t), can create inversion layers in the body region 110 sufficient toa form the conductive channel sufficient to permit conduction betweenthe source 119 and drain 120 regions.

As described in more detail below in connection with the truth table ofFIG. 2, if the floating gate, 114, is not charged, the voltagethreshold, V_(t), necessary to form a conductive channel in the bodyregion 110 will be lower. If, as illustrated further in connection withthe truth table of FIG. 2, both a potential applied to the first input,e.g., control gate 118, and potential applied to the second input, e.g.,back gate 104, are low, then a logical output of “1” will be produced.However, when the floating gate, 114, is in this uncharged state, a highpotential applied to either the first input, e.g., control gate 118, orthe second input, e.g., back gate 104, will be sufficient toindividually turn the transistor structure of cell 101 “ON” producingconduction between the source 119 and the drain 120 such that a logicaloutput of “0” will be produced. As the reader will appreciate, when thefloating gate, 114, is in this uncharged state, a high potential appliedto both the first input, e.g., control gate 118, and the second input,e.g., back gate 104, will turn the transistor structure of cell 101 “ON”producing conduction between the source 119 and the drain 120 such thata logical output of “0” will be produced, e.g., a NOR logical operationperformed.

Alternatively, as described in more detail below in connection with thetruth table of FIG. 2, if the floating gate, 114, is charged, i.e.,charged negative, the voltage threshold, V_(t), necessary to formconductive channels in the body region 110 will be high. If, asillustrated further in connection with the truth table of FIG. 2, both apotential applied to the first input, e.g., control gate 118, andpotential applied to the second input, e.g., back gate 104, are low,then a logical output of “1” will be produced, e.g., the cell will notturn “ON”. That is, the potential applied to the first and/or secondinput, e.g., control gate 118 and back gate 104, will not be sufficientto overcome the voltage threshold, V_(t), and create a channel toproduce conduction between source region 119 and drain region 120.According to one or more embodiments, either high input alone to thefirst or second input, e.g., control gate 118 or back gate 104, will notby itself be sufficient to overcome, V_(t), sufficient to turn on thetransistor structure of cell 101. The charge state of the floating gate,114, will control and the potential applied to the back gate, 104, willmodulate the threshold voltage, e.g., V_(t).

Hence, according to one or more embodiments, if the floating gate 114 ischarged negative, e.g., a second programmed state, then an input to onlyone of the control gate 118 or the back gate 104, e.g., first and secondinputs, is insufficient to individually turn the transistor structure ofcell 101 “ON” and produce conduction between the source 119 and thedrain 120 such that a logical output of “1” will be produced. When thefloating gate is charged negative, e.g., a second programmed state, thenonly a positive input signal applied to both the control gate 118 andthe back gate 104, e.g., first and second inputs, will be sufficient toturn the transistor structure of cell 101 “ON” and produce conductionbetween the source 119 and the drain 120 such that a logical output of“0” will be produced. e.g., a logical NAND operation. That is, the cellstructure 101 will only conduct when both inputs, e.g., control gate 118and back gate 104 are both high.

Again, as noted above, the charge state, e.g., either charged or notcharged will control the voltage threshold V_(t), such that the cellstructure 101 will turn “ON” in the charged state if and only if boththe first and second inputs, e.g., control gate 118 and back gate 104,are at a positive or high logic level. The voltage threshold, accordingto one or more embodiments, can be determined by appropriate doping,e.g., approximately 5×10¹⁵/cm³, and thickness, e.g. width of the bodyregion 110 approximately 100 Å, stack and gate insulator thickness, workfunctions of the gate materials, and voltages of the logic levels, e.g.,charge on the third logic input (floating gate 114), potential appliedto the first logic input (control gate 118), and potential applied tothe second logic input (back gate 104).

FIG. 1C illustrates an embodiment of the cell structure comprising atransistor with a first gate, a second gate, and a third gate, e.g., acontrol gate, a back gate, and a floating gate, configurable as a memoryin logic cell. The transistor in FIG. 1C has a silicon channel 110 withtwo source/drain regions 130. The transistor operates by having avoltage potential across the silicon channel 110 between the sourceregion 130 and the drain region 130 to cause the transistor to turn onand allow electrical flow through the body.

In one or more embodiments, a self-aligned back gate, control gate, andfloating gate cell is used as a programmable logic device. In one ormore embodiments, the back gate 104 is on an oxide layer 102 with LOCOSregions 106 and has two oxidized regions 128 on the ends of the backgate 104 to define it thickness. In FIG. 1C, alternating nitride andoxide layers form the stack containing the control gate 118 and thefloating gate 114. A gate oxide layer 112 separates the silicon channel110 from the floating gate 114. In one or more embodiments, the floatinggate 114 can be formed from a dope poly-silicon or a nitride. A controlgate dielectric 116 is between the control gate 118 and the floatinggate 114. In some embodiments, the control gate 118 is formed frompoly-silicon. An oxide dielectric layer 120 caps the control gate 118.The silicon channel's thickness is defined by oxide spacers 124 that areon each side of the silicon channel 110.

FIG. 2 illustrates a truth table for various first, second, and thirdlogical inputs to the memory in logic cell shown in FIGS. 3A and 3B. Thetable in FIG. 2 shows a truth table for various first, second, and thirdlogical inputs to the programmable computing array 300 shown in FIGS. 3Aand 3B. The memory in logic cells, 301-1, 301-2, . . . , 301-T, arecoupled to the p-type transistors, 312-1, . . . , 312-T, to output thelogical value from the array. In this embodiment, the memory in logiccells, 301-1, 301-2, . . . , 301-T in the programmable computing array300 function with the p-type transistors, 312-1, . . . , 312-T, toperform NAND and/or NOR logic functions, depending on the state, chargedor uncharged, of the floating gate, 308 (third input) and for variouspotential (logic inputs) to the control gate, 306, and the back gate 304(first and second logical inputs).

The third logical input is the floating gate, 308 (FG), to each of thememory in logic cells, 301-1, 301-2, . . . , 301-T, represented aseither charged (−) or uncharged (0V) in column 234. In a first portionof the truth table 231, the truth table illustrates the logic performedby the output of the programmable computing array 300 when the floatinggate, 308 (FG), is charged (−). In a second portion of the truth table232, the truth table illustrates the logic performed by output of theprogrammable computing array 300 when the floating gate, 308 (FG), isuncharged (0V).

As shown in FIG. 3B, the first logical input is the potential applied tothe control gate, 306 (CG), of the memory in logic cells, 301-1, 301-2,. . . , 301-T, represented as a logic “1” or logic “0” value (column236) depending on a high or low potential applied to the control gate,306 (CG), along a row, R-1, R-2, . . . , R-N respectively, of memory inlogic cells, 301-1, 301-2, . . . , 301-T. The second logical input isthe potential applied to the back gate, 304 (BG), of the memory in logiccells, 301-1, 301-2, . . . , 301-T, represented as a logic “1” or logic“0” value (column 238) depending on a high or low potential applied tothe back gate, 304 (BG), along a column, CL-1, CL-2, . . . , CL-M, ofmemory in logic cells, 301-1, 301-2, . . . , 301-T.

As shown in the first portion of the truth table 231, when the floatinggate, 308 (FG) is charged negative (−), the system functions as a firstlogical gate type to perform a first logical operation based on thefirst and the second logical inputs (CU and BG). That is, the output ofthe memory in logic cells, 301-1, 301-2, . . . , 301-T, function toperform a “NAND” logic operation such that only when a logical “1” valueis applied to both the first logical input, e.g., the control gate 306(CG), and to the second logical input, e.g., the back gate 304 (BG),will an output, as reflected in column 240, be a logical “0” value andthe device state 242 is “ON”.

As shown in the second portion of the truth table 232, when the floatinggate, 308 (FG) is uncharged (0V), the system functions as a secondlogical gate type to perform a second logical operation based on thefirst and the second logical inputs (CG and BG). That is, the output ofthe memory in logic cells, 301-1, 301-2, . . . , 301-T, function toperform a “NOR” logic function such that when a logical “1” value isapplied to either the first logical input, e.g., the control gate 306(CG), or to the second logical input, e.g., the back gate 304 (BG), anoutput, as reflected in column 240, be a logical “0” value and thedevice state 242 is “ON”. Here, only when a logical “0” value is appliedto both the first logical input, e.g., the control gate 306 (CG), and tothe second logical input, e.g., the second control gate 304 (BG), willan output, as reflected in column 240, be a logical “1” value.

FIGS. 3A and 3B illustrate a programmable computing array implementingembodiments of the memory in logic cells described in connection withFIGS. 1A-2. As shown in the embodiment of FIG. 3A the programmablecomputing array 300 includes a number of memory in logic cells, 301-1,301-2, . . . , 301-T, arranged in a matrix of columns, CL-1, CL-2, CL-M,and rows, R-1, R-2, . . . , R-N. Each of the memory in logic cells,301-1, 301-2, . . . , 301-T, in the programmable computing array 300 caninclude the structure and operation described in connection with FIGS.1A-3 above. That is, the memory in logic cells, 301-1, 301-2, . . . ,301-T, include a floating gate, 308, (third input, FG) which are a sideof a narrow, e.g., less than 100 Angstroms, semiconductor channel. Acontrol gate, 306, (first input, CG) is formed opposing the floatinggate, 308. A back gate, 304, (second input, BG) is formed on theopposite side of the channel region than the control gate 306 and thefloating gate 308.

The semiconductor channel region can be formed upon a substrateaccording to known semiconductor fabrication techniques and include anynumber of suitable number of semiconductor materials. The semiconductorchannel region is doped so as to form a source region 305, a body region307, and a drain region 303. In one embodiment, the source regions 305and drain regions 303 include n-type doping and the body regions 307include p-type doping so as to form n-channel transistor structures tothe memory in logic cells, 301-1, 301-2, . . . , 301-T. As shown in theembodiment of FIG. 3A, a contact 318-T is connected to the drain region303 for each of the semiconductor channel regions. A conductive rowline, e.g., 320-1, . . . , 320-T, along each row, R-1, . . . , R-N, isconnected to contacts 318-1, . . . , 318-T. As shown in the embodimentof FIG. 3A, each of the row lines, 320-1, . . . , 320-T, are connectedto a p-type transistor, 312-1, . . . , 312-T, in order to form aninverter output.

As explained above, each of the semiconductor channel regions in theprogrammable computing array 300 are sufficiently lightly doped, e.g.,have a doping concentration of approximately 5×10¹⁵/cm³, such that avoltage threshold V_(t) for the channel region depends on the charge orabsence of charge stored on the floating gate 308 and is modulated by agate potential applied to the back gate 304 as a second input. In thismanner, the voltage threshold, V_(t), of the channel regions in theprogrammable computing array 300 are dependent on the charge on thefloating gate in each cell.

In connection with the truth table of FIG. 2, the logic that each of thecells, 301-1, 301-2, . . . , 301-T, outputs is dependent on the chargeon the floating gate, e.g., 308, as a third input, and is modulated by apotential applied to a potential applied to a back gate, 304, as asecond input. The memory in logic cells, 301-1, 301-2, . . . , 301-T, inthe programmable computing array 300 are coupled to the p-typetransistors, 312-1, . . . , 312-T, which are supplied with V_(dd)voltage potential, to output the logical value for the cell. When thememory in logic cells, 301-1, 301-2, . . . , 301-T, in the programmablecomputing array 300 are turn “ON” and conducting, the row lines, 320-1,. . . , 320-T are pulled down to ground and the logical output for thecell is “0”. When the memory in logic cells, 301-1, 301-2, . . . ,301-T, in the programmable computing array 300 are turn “OFF” by notconducting, the row lines, 320-1, . . . , 320-T are pulled up to V_(dd)by the p-type transistors, 312-1, . . . , 312-T, therefore the logicaloutput for the cell is “1”. Hence, the programmable computing array 300can function with the p-type transistors, 312-1, . . . , 312-T, toperform NAND and/or NOR logic functions, depending on the state, chargedor uncharged, of the floating gate, 308 (third logical input) forvarious potential (logic inputs) to the control gate, 306, (firstlogical input) and to the back gate, 304, (second logical input).

Each of the memory in logic cells, e.g., 301-1, in the programmablecomputing array 300 can include the structure and operation described inconnection with FIGS. 1A-2 above, e.g., can be programmed to perform aNOR 316 and a NAND 314 logical operation, etc.

As explained above, the logic function of a given cell 301-1, . . . ,301-T the array 300 is dependent on the charge on the floating gate,e.g., third input, and the potential applied to the back gate, e.g.,second input, associated with the given cell 301-1, . . . , 301-T. Amemory in logic cell 301-1, . . . , 301-T in the array 300 performs aNAND logic function when the floating gates of a cell 301-1, . . . ,301-T are charged negative, as described in connection with FIG. 2. Amemory in logic cell 301-1, . . . , 301-T in the array 300 performs aNOR logic function when the floating gates are uncharged, as describedin connection with FIG. 2.

FIG. 4 illustrates an embodiment of a memory in logic computing systemwhere multiple programmable computing arrays, e.g., 402-1, . . . ,402-M, are coupled together via a programmable routing structure, e.g.,410 and 412. The programmable computing arrays 402-1, . . . , 402-Mimplement embodiments of the memory in logic cells having the structureand operation described in connection with FIGS. 1A-3. That is, thefloating gate, third input, and control gate, first input, and the backgate, second input, to the memory in logic cells of the arrays, 402-1, .. . , 402-M, are configurable to allow the memory in logic cells toserve as NAND or NOR logic gates, etc.

As shown in the embodiment of FIG. 4, the programmable computing arrays,402-1, . . . , 402-M, can be connected through programmable routingstructures 410, 412, etc. The programmable routing structures 410, 412,etc., can receive an output on row lines, e.g., 406-1, from a firstprogrammable computing array, e.g., 402-1, and connect this signal on toother programmable computing arrays, effectively linking different logicblocks together to allow the logic functions to be combined. As shown inthe embodiment of FIG. 4, the programmable routing structures 410, 412can include a matrix of pass transistors, e.g., two pass transistors414, 416, etc., and may themselves include programmable memory in logiccells, e.g., 418 and 420 as the same have been described herein, tocontrol the gates of such pass transistors, e.g., 414, 416, etc.

Hence, the programmable routing structures 410, 412, etc., can beprogrammed using programmable logic devices 418, 420, such as the memoryin logic cells described herein, to combine the logic signals from thememory in logic cells in the programmable computing arrays 402-1, . . ., 402-M, in a variety of ways. As shown in the embodiment of FIG. 4, alogic signal from a memory in logic cell in array 402-1 can be allowedto pass through the programmable routing structure 410 by addressingpass transistor 414 with address lines 422 and with the programmablelogic device 418 to output a logical signal on line 406-1. Likewiseaddress signals 422 can address pass transistor 416 together with theprogrammable logic device 420 to output a logical signal on line 406-Oas an input to programmable computing array 402-M.

In this manner, an output of a first memory in logic cell, e.g., 404-1,in array 402-1, can be programmably connected as a first input, via line406-O, to a second memory in logic cell, e.g., 404-N in array 402-M, viaone or more programmable routing circuits, e.g., 410, 412, etc.Moreover, output of a third memory in logic cell can be connected as asecond input, e.g., 406-P, to the second memory in logic cell, e.g.,402-M. As the reader will appreciate the second memory in logic cell,402-M, can be configured to perform a third logical operation. Althoughthe example embodiment of FIG. 4 illustrates an output from a firstprogrammable computing array 402-1 as an output from a memory in logiccell 404-1 configured to perform a NAND logic function, and the input toa programmable computing array 402-M as being input to a memory in logiccell 404-N similarly configured to perform a NAND logic function,embodiments are not limited to this example.

As one of skill in the art will appreciate upon reading this disclosure,one or more embodiments include an output from a first programmablearray, having cells configured to perform the first logical operation,being provided as an input to a second programmable array, having cellsconfigured to perform as second, different logical operation.Additionally, a first output from a first programmable array can beprovided as an input to a second programmable array and a second outputfrom the first programmable array can be provided as an input to a thirdprogrammable array via the programmable routing structures, 410, 412,etc., described herein.

FIGS. 5A-5H illustrate an embodiment of the process steps in forming atransistor with a floating gate, a control gate, and back gate to beconfigured as a memory in logic cell in a programmable computing array.In various embodiments, the process in FIGS. 5A-5H creates aself-aligned back gate, control gate, and floating gate in thetransistor. In one embodiment a method of forming a combined memory inlogic cell comprises forming a first gate structure surrounded by adielectric on a first substrate according to a complementary metal oxidesemiconductor (CMOS) process, flipping, e.g., inverting, the substrate,and bonding the substrate to a second substrate, e.g., donor wafer, suchthat the first gate structure is positioned as an isolated back gate toserve as a logical input to a memory in logic cell. The method alsoincludes forming a channel region by planarizing the flipped substrateand forming a floating gate and a control gate stack above the channelregion according to a CMOS process. A patterning and etching techniquecan also occur around the floating gate and the control gate stack intothe substrate to align the back gate with the stack and a patterning andetching technique can occur according to a self aligned CMOS techniquesuch that the floating gate, the control gate, and the back gate arealigned in width opposing the channel region. Electrical contacts to thechannel region, the control gate, and the back gate can be formed suchthat the control gate, the back gate, and the floating gate serve as afirst, a second, and a third logical input to a memory in logic cellstructure.

FIG. 5A shows a back gate 504 surrounded by a gate oxide 502 with LOCOS506 as a side cap formed on a first substrate 500. A first gatestructure with poly-silicon interconnect is formed using complimentarymetal on oxide (CMOS) processing employing oxide isolation and chemicalmechanical planarization (CMP) for planarization across the wafer. Thefirst structure is comprised of a back gate 504 which is surrounded by agate oxide 502 and LOCOS 506 as side caps.

In FIG. 5B, a step in forming a transistor with a floating gate, acontrol gate, and back gate is shown. The back gate structure 504, gateoxide 502, first substrate 500, and LOCOS 506 are then flipped and thegate oxide 502 bonded to a second substrate, e.g., donor wafer, 508. Thesecond substrate 508 is present during the process steps to form thetransistor and removed by steps known in industry once the functionalfeatures of the transistor are formed. This orients the back gate 504 onthe bottom of the structure and allows for further processing steps tobe completed on the structure.

In FIG. 5C, the silicon layer 510 forming the body of the transistor isfabricated. The first substrate 500 is polished by CMP to form thesilicon layer 510. The polishing of the first substrate to a desiredthickness forms the body region for the transistor, which is where thechannel is formed that allows conduction between the source and drainregions in the transistor.

In FIG. 5D, alternating nitride and oxide layers are deposited andetched to pattern and form the stack containing the control gate and thefloating gate. A gate oxide layer 512 is formed on the silicon channellayer 510. The floating gate 514 is formed on the gate oxide layer 512.In various embodiments, the floating gate can be formed from a dopepoly-silicon or a nitride. A control gate dielectric 516 is formed onthe floating gate, which in various embodiments could be an oxide. Thecontrol gate 518 is formed with the control gate dielectric 516 betweenthe control gate 518 and the floating gate 514. In some embodiments, thecontrol gate 518 is formed from poly-silicon. An oxide dielectric layer520 is form on the control gate 518 and the stack is capped with anitride layer 522.

In FIG. 5E, the silicon channel 510 is etched to a thickness that isdetermined by the desired operating characteristics and thresholdvoltages. The silicon channel's thickness is defined by oxide spacers524 that are formed on each side of the stack. Oxide spacers 524 areadded to each side of the pillar to allow the silicon channel 510 to beetched to a thickness where the outer edge of the two oxide spacers 524define the thickness of the silicon channel 510.

In FIG. 5F, the back gate 504 is etched to a thickness defined bynitride spacers 526. The back gate thickness is determined by selfaligning the back gate 504 with the thickness of the nitride spacers526. Nitride spacers 526 are added to each side of the stack, wherebythe outer edge of the nitride spacers define the thickness of the backgate 504. The oxide layer 502 that is between the back gate 504 and thesilicon channel 510 is etched along with the portion of the back gate504 region outside of the edges of the nitride spacers 526. This etchingforms the operational portion of the back gate 504 and defines thethickness of the back gate 504.

In FIG. 5G, the operational back gate thickness is adjusted. The backgate 504 is oxidized to undercut and insulate. An oxidized region 528 isformed on each end of the back gate 504. In some embodiments, theoxidation process narrows the back gate to the same thickness as thecontrol gate 518 and the floating gate 514.

In FIG. 5H, the source and drain regions are defined for the transistor.The nitride spacers 526 are removed and doped poly-silicon is depositedin the void left from the etching that defined the back gate 504thickness and the removal of the nitride spacers 526. The source anddrain regions 530 are then defined by the poly-silicon area that is incontact with the ends of the silicon channel 510.

Conclusion

Methods, devices, and systems for a memory in logic cell are provided.One or more embodiments include using a cell structure having a firstgate, a second gate, and a third gate, e.g., a control gate, a backgate, and a floating gate, as a memory in logic cell. The methodincludes programming the floating gate to a first state to cause thememory in logic cell to operate as a first logic gate type. The methodfurther includes programming the floating gate to a second state tocause the memory in logic cell to operate as a second logic gate type.

Although specific embodiments have been illustrated and describedherein, those of ordinary skill in the art will appreciate that anarrangement calculated to achieve the same results can be substitutedfor the specific embodiments shown. This disclosure is intended to coveradaptations or variations of various embodiments of the presentdisclosure. It is to be understood that the above description has beenmade in an illustrative fashion, and not a restrictive one. Combinationof the above embodiments, and other embodiments not specificallydescribed herein will be apparent to those of skill in the art uponreviewing the above description. The scope of the various embodiments ofthe present disclosure includes other applications in which the abovestructures and methods are used. Therefore, the scope of variousembodiments of the present disclosure should be determined withreference to the appended claims, along with the full range ofequivalents to which such claims are entitled.

In the foregoing Detailed Description, various features are groupedtogether in a single embodiment for the purpose of streamlining thedisclosure. This method of disclosure is not to be interpreted asreflecting an intention that the disclosed embodiments of the presentdisclosure have to use more features than are expressly recited in eachclaim. Rather, as the following claims reflect, inventive subject matterlies in less than all features of a single disclosed embodiment. Thus,the following claims are hereby incorporated into the DetailedDescription, with each claim standing on its own as a separateembodiment.

1. A method of forming a memory in logic cell, comprising: forming afirst gate structure surrounded by a dielectric on a substrate accordingto a complementary metal oxide semiconductor (CMOS) process; flippingthe substrate such that the first gate structure is positioned as anisolated back gate to serve as a logical input to the memory in logiccell; forming a channel region by planarizing the flipped substrate; andforming a floating gate and a control gate stack above the channelregion according to a CMOS process.
 2. The method of claim 1, whereinthe method includes patterning and etching around the floating gate andthe control gate stack into the substrate to align the back gate withthe stack.
 3. The method of claim 2, wherein the method includespatterning and etching according to a self aligned CMOS technique suchthat the floating gate, the control gate, and the back gate are alignedin width opposing the channel region.
 4. The method of claim 3, whereinthe method includes forming electrical contacts to the channel region,the control gate, and the back gate such that the floating gate, thecontrol gate, and the back gate serve as a first, a second, and a thirdlogical input to the memory in logic cell structure.
 5. The method ofclaim 2, wherein the method includes forming a first oxide spacer on afirst side of the stack and a second oxide spacer on a second side ofthe stack.
 6. The method of claim 5, wherein the method includesremoving a portion of a floating gate dielectric and a portion of thesilicon channel to define a first end of the silicon channel verticallyaligned with the first oxide spacer and a second end of the siliconchannel vertically aligned with the second oxide spacer.
 7. The methodof claim 6, wherein the method includes forming a first nitride spaceron the first side of the stack and a second nitride spacer on the secondside of the stack.
 8. The method of claim 7, wherein the method includesremoving a portion of the back gate to define a first end of the backgate vertically aligned with the first nitride oxide spacer and a secondend of the back gate vertically aligned with the second nitride spacer.9. The method of claim 8, wherein the method includes oxidizing the backgate to undercut the back gate in alignment with a width of the floatinggate.
 10. A method of forming a memory in logic cell, comprising:forming a first gate structure surrounded by a dielectric on a firstsubstrate according to a complementary metal oxide semiconductor (CMOS)process; inverting the first substrate and bonding the first substrateto a second substrate such that the first gate structure is positionedas an isolated back gate to serve as a first logical input to the memoryin logic cell; forming a channel region by planarizing the invertedfirst substrate; forming a stack that includes a floating gate and acontrol gate over the channel region; and aligning the stack with theback gate.
 11. The method of claim 10, wherein the method includesforming electrical contacts to the channel region, the control gate, andthe back gate such that the control gate serves as a second logicalinput to the memory in logic cell.
 12. The method of claim 11, includingforming the memory in logic cell such that the memory in logic cellprovides different logical outputs, in response to differentcombinations of the first and second logical inputs, for a givenprogrammed state of the floating gate.
 13. A method of forming a memoryin logic cell, comprising: forming a first gate structure surrounded bya dielectric on a substrate according to a complementary metal oxidesemiconductor (CMOS) process; flipping the substrate such that the firstgate structure is positioned as an isolated back gate to serve as afirst logical input to the memory in logic cell; and forming a stackincluding a control gate and a floating gate over a channel region onthe substrate; wherein the control gate is formed to receive a firstlogical input and the back gate is formed to receive a second logicalinput; and wherein the memory in logic cell is formed to perform a firstlogical operation based on the first logical input and the secondlogical input when the floating gate is programmed to a first state andto perform a second logical operation based on the first logical inputand the second logical input when the floating gate is programmed to asecond state.
 14. The method of claim 13, including forming the memoryin logic cell such that the memory in logic cell performs a NAND logicfunction in response to the first state of the floating gate and a NORlogic function in response to the second state of the floating gate. 15.The method of claim 13, including forming the memory in logic cell suchthat it provides different logical outputs, based on differentcombinations of the first and second logical inputs, for a givenprogrammed state of the floating gate.
 16. The method of claim 13,including forming the floating gate to receive a third logical input.17. The method of claim 13, including forming the memory in logic cellsuch that an output of the cell serves as an input to a second memory inlogic cell.